This work examines the phenomenon of programming (imprinting) of steroid metabolizing enzymes in the rat in an effort to determine the role of the endocrine system in this process. Initial studies examine the dual localization of 5 alpha-reductase in the microsomes and nuclei of the immature and mature rat, both male and female. The effects of early neonatal exposure to testosterone propionate or DES on normal programming of adult enzyme activities or on enzyme responsiveness to hormonal stimulation are examined. The ability of pure PCB isomers to alter normal programming of hepatic steroid metabolizing enzymes when administered prenatally is also investigated. Future studies will attempt to further elucidate the role of the hypothalamo-pituitary-gonadal axis in the regulation and modulation of hepatic 5 alpha-reductase and 16 alpha-hydroxylase activities.